In recent years, gallium nitride based compound semiconductor light-emitting devices have drawn attention as short wavelength light-emitting devices. The gallium nitride based compound semiconductor light-emitting device is formed on various kinds of substrates, such as a sapphire single crystal substrate, an oxide substrate, and a III-V group compound substrate, by a metal organic chemical vapor deposition (MOCVD) method or a molecular beam epitaxy (MBE) method.
The gallium nitride based compound semiconductor light-emitting device is characterized in that a small amount of current is diffused in the horizontal direction. Therefore, a current is applied to only a semiconductor immediately below an electrode, and light emitted from a light-emitting layer immediately below the electrode is shielded by the electrode, which makes it difficult to emit light from the light-emitting device to the outside. In addition, in the light-emitting device, a transparent electrode is generally used as a positive electrode, and light is emitted to the outside through the positive electrode.
The positive electrode composed of the transparent electrode is formed of a known conductive material, such as Ni/Au or ITO(In2O3—SnO2). Metallic materials, such as Ni/Au, have low contact resistance with a p-type semiconductor layer, but have low light transmittance. On the other hand, oxides, such as ITO, have high a light transmittance, but have a high contact resistance.
For this reason, in the related art, the positive electrode used for the gallium nitride based compound semiconductor light-emitting device is formed by combining a contact metal layer with a metal oxide layer formed of a high conductive material, such as ITO (for example, Patent Document 1, i.e. JP-A-9-129919).
The contact metal layer has been formed of a metallic material having a large work function, such as Pt or Rh, in order to reduce contact resistance with the p-type semiconductor layer.
However, in the gallium nitride based compound semiconductor light-emitting device disclosed in Patent Document 1, the contact metal layer used for the positive electrode can reduce the contact resistance with the p-type semiconductor layer, but it is difficult to obtain sufficient light emission efficiency since the transmittance of the contact metal layer is low, which results in low emission power.
A method of increasing the transmittance of each layer has been proposed to improve light emission efficiency and thus increase the emission power in the gallium nitride based compound semiconductor light-emitting device. In addition, another method has been proposed which improves the light emission efficiency by forming a rough emission surface to emit light at various angles (for example, Patent Document 2, i.e. JP-A-6-291368).
In the gallium nitride based compound semiconductor light-emitting device disclosed in Patent Document 2, the formation of the rough emission surface enables the light-emitting layer to have a refractive index of about 2.5 that is considerably higher than that of air, which is 1, and a small threshold angle of about 25°. Therefore, it is possible to prevent no light from being emitted to the outside due to the repeated reflection and absorption of light in the crystal. As a result, the light emission efficiency is improved.
However, in the gallium nitride based compound semiconductor light-emitting device disclosed in Patent Document 2, the formation of the rough emission surface makes it possible to improve the light emission efficiency, but during a process of forming the rough emission surface, the rough emission surface is damaged, which results in an increase in the contact resistance with the electrode.
In order to solve the problem of the increase in the contact resistance, a light-emitting device having a low contact resistance has been proposed in which a rough emission surface is formed on a gallium nitride based compound semiconductor light-emitting device, a metal layer including a Mg layer and a Au layer is provided in the vicinity of the surface of a p-type semiconductor layer, and a heat treatment is performed to reduce the contact resistance (for example, Patent Document 3, i.e. JP-A-2000-196152).
However, in the gallium nitride based compound semiconductor light-emitting device disclosed in Patent Document 3, after the metal layer including the Mg layer and the Au layer is formed, the heat treatment needs to be performed, and the metal layer needs to be removed. As a result, the number of processes significantly increases, and thus manufacturing costs increase. In addition, it is necessary to use a strong acid, such as aqua regia, in order to remove the Au layer. In this case, there is a fear that the surface of the gallium nitride based compound semiconductor will be damaged.
The invention has been made in order to solve the above problems, and an object of the invention is to provide a gallium nitride based compound semiconductor light-emitting device capable of obtaining high light emission efficiency by increasing the dopant concentration of a transparent conductive oxide film, without using a contact metal layer having a low light transmittance for a positive electrode, and reducing contact resistance with a p-type semiconductor layer to lower a driving voltage Vf, and a method of manufacturing the same.
Another object of the invention is to provide a gallium nitride based compound semiconductor light-emitting device capable of reducing contact resistance between a transparent conductive oxide film and a p-type semiconductor layer having an uneven surface on at least a portion thereof to reduce a driving voltage Vf and improving light emission efficiency, by increasing the dopant concentration of the transparent conductive oxide film, without using a contact metal layer having low light transmittance, and a method of manufacturing the same.